1. Field of the Invention
The present invention relates to an R—Fe—B based microcrystalline high-density magnet produced by an HDDR process and a method for producing such a magnet.
2. Description of the Related Art
An R—Fe—B based rare-earth magnet (where R is a rare-earth element, Fe is iron, and B is boron) is a typical high-performance permanent magnet, has a structure including, as a main phase, an R2Fe14B phase, which is a ternary tetragonal compound, and exhibits excellent magnet performance. Such R—Fe—B based rare-earth magnets are roughly classifiable into sintered magnets and bonded magnets. A sintered magnet is produced by compacting a fine powder of an R—Fe—B based magnet alloy (with a mean particle size of several μm) with a press machine and then sintering the resultant compact. On the other hand, a bonded magnet is produced by compression-molding or injection-molding a mixture (i.e., a compound) of a powder of an R—Fe—B based magnet alloy (with particle sizes of about 100 μm) and a binder resin.
The sintered magnet is made of a powder with relatively small particle sizes, and therefore, the respective powder particles thereof exhibit magnetic anisotropy. For that reason, an aligning magnetic field is applied to the powder being compacted by the press machine, thereby making a powder compact in which the powder particles are aligned with the direction of the magnetic field.
The powder compact obtained in this manner is then sintered normally at a temperature of 1,000° C. to 1,200° C. and then heat-treated if necessary to be a permanent magnet. In the sintering process, the atmosphere is often a vacuum atmosphere or an inert atmosphere to reduce the oxidation of the rare-earth element.
To make the bonded magnet exhibit magnetic anisotropy on the other hand, the hard magnetic phases in the powder particles used should have their easy magnetization axes aligned in one direction. Also, to achieve coercivity to a practically required level, the crystal grain size of the hard magnetic phases that form the powder particles should be reduced to around the single domain critical size. For these reasons, to produce a good anisotropic bonded magnet, a rare-earth alloy powder that satisfies all of these conditions needs to be obtained.
To make a rare-earth alloy powder for an anisotropic bonded magnet, an HDDR (hydrogenation-disproportionation-desorption-recombination) process is generally adopted. The “HDDR” means a process in which hydrogenation, disproportionation, desorption and recombination are carried out in this order. In the known HDDR process, an ingot or powder of an R—Fe—B based alloy is maintained at a temperature of 500° C. to 1,000° C. within an H2 gas atmosphere or a mixture of an H2 gas and an inert gas so as to occlude hydrogen into the ingot or the powder. After that, the desorption process is carried out at the temperature of 500° C. to 1,000° C. until either a vacuum atmosphere with an H2 pressure of 13 Pa or less or an inert atmosphere with an H2 partial pressure of 13 Pa is created and then a cooling process is carried out.
In this process, the reactions typically advance in the following manner. Specifically, as a result of a heat treatment process for producing the hydrogen occlusion, the hydrogenation and disproportionation reactions (which are collectively referred to as “HD reactions” that may be represented by the chemical reaction formula: Nd2Fe14B+2H2→2NdH2+12Fe+Fe2B) advance to form a fine structure. Thereafter, by carrying out another heat treatment process to produce the desorption, the desorption and disproportionation reactions (which are collectively referred to as “DR reactions” that may be represented by the chemical reaction formula: 2NdH2+12Fe+Fe2B→Nd2Fe14B+2H2) are produced to make an alloy with very fine R2Fe14B crystalline phases.
An R—Fe—B based alloy powder, produced by such an HDDR process, exhibits high coercivity and has magnetic anisotropy. The alloy powder has such properties because the metallurgical structure thereof substantially becomes an aggregate structure of crystals with very small sizes of 0.1 μm to 1 μm. Also, if the reaction conditions and composition are selected appropriately, the easy magnetization axes of the crystals will be aligned in one direction, too. More specifically, the high coercivity is achieved because the sizes of the fine crystal grains, obtained by the HDDR process, are close to the single domain critical size of a tetragonal R2Fe14B based compound. The aggregate structure of those fine crystals of the tetragonal R2Fe14B based compound will be referred to herein as a “recrystallization texture”. Methods of making an R—Fe—B based alloy powder having the recrystallization texture by the HDDR process are disclosed in Patent Documents Nos. 1 and 2, for example.
A magnetic powder made by the HDDR process (which will be referred to herein as an “HDDR powder”) is normally mixed with a binder resin (which is also simply referred to as a “binder”) to make a compound, which is then either compression-molded or injection-molded under a magnetic field, thereby producing an anisotropic bonded magnet. The HDDR powder will usually aggregate after the HDDR process. Thus, to use the powder to make an anisotropic bonded magnet, the aggregate structure is broken down into the powder again. For example, according to Patent Document No. 1, the magnet powder obtained preferably has a particle size of 2 μm to 500 μm. In Example 1 of that document, an aggregate structure obtained by subjecting a powder with a mean particle size of 3.7 μm to the HDDR process is crushed in a mortar to obtain a powder with a mean particle size of 5.8 μm. Thereafter, the powder is mixed with a bismaleimide triazine resin and then the compound is compression-molded to make a bonded magnet.
On the other hand, various techniques for making a microcrystalline high-density magnet by taking advantage of the HDDR process have also been proposed. According to one of those techniques, an HDDR magnetic powder is aligned and then turned into a bulk material by a hot compaction process such as a hot pressing process or a hot isostatic pressing (HIP) process. Such a technique is disclosed in Patent Documents Nos. 3 to 8, for example. By adopting a hot compaction process, the density of the powder can be increased at temperatures of 600° C. to 900° C., which are lower than the sintering temperature. As a result, a bulk magnet can be produced with the recrystallization texture of the HDDR powder maintained.
Meanwhile, according to Patent Document No. 9, an alloy that has been subjected to HD reactions and a desorption reaction to such a degree as to produce no coercivity yet is compacted under a magnetic field, and the resultant powder compact is subjected to DR reactions and then hot pressing. In this manner, the demagnetization process can be omitted when the powder needs to be compacted under a magnetic field and yet the anisotropy can be increased, according to Patent Document No. 9.
Also, according to the method disclosed in Patent Document No. 10, an R—Fe—B based alloy that has been prepared by melting materials in an induction melting furnace is subjected to a solution treatment, if necessary, cooled, and then pulverized into a coarse powder. The powder is further pulverized finely to a size of 1 μm to 10 μm using a jet mill, for example, and then compacted under a magnetic field. Thereafter, the green compact is sintered at a temperature of 1,000° C. to 1,140° C. within either a high vacuum or an inert atmosphere. Then, the sintered compact is heated to a temperature of 600° C. to 1,100° C. within a hydrogen atmosphere and then thermally treated within a high vacuum, thereby reducing the size of the main phase to 0.01 μm to 1 μm.
Furthermore, according to the method disclosed in Patent Document No. 11, first, an alloy that has been subjected to a solution treatment process is pulverized to a particle size of less than 10 μm with a pulverizer such as a jet mill, and then the powder is compacted under a magnetic field to obtain a powder compact. Then, the powder compact is treated at a temperature of 600° C. to 1,000° C. within hydrogen and then at a temperature of 1,000° C. to 1,150° C. This series of processes carried out on the powder compact corresponds to the HDDR process. In this case, however, the temperature of the DR process is higher than the rest of the process. According to the method disclosed in Patent Document No. 11, the sintering process is advanced by the DR process at the higher temperature, and therefore, the powder compact can be sintered as densely as it has been. Patent Document No. 11 says that the sintering process should be carried out at a temperature of at least 1,000° C. to make a sintered body with high density.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 1-132106        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2-4901        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2-39503        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 4-245403        Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 4-246803        Patent Document No. 6: Japanese Patent Application Laid-Open Publication No. 4-247604        Patent Document No. 7: Japanese Patent Application Laid-Open Publication No. 4-253304        Patent Document No. 8: Japanese Patent Application Laid-Open Publication No. 11-195548        Patent Document No. 9: Japanese Patent Application Laid-Open Publication No. 2001-85256        Patent Document No. 10: Japanese Patent Application Laid-Open Publication No. 4-165012        Patent Document No. 11: Japanese Patent Application Laid-Open Publication No. 6-112027        
It is well known to those skilled in the art that crystals at the uppermost surface of an Nd2Fe14B-type magnet have no coercivity. A sintered magnet includes a higher percentage of Nd2Fe14B phase as a hard magnetic phase, and therefore achieves better magnetic properties, than a bonded magnet. However, a sintered magnet normally has a crystal grain size of approximately 3 to 10 μm. Thus, it is also known that if a sintered magnet is machined to a size of 3 mm or less, for example, the effect of that uppermost surface portion with no coercivity will manifest itself and cause significant deterioration in its properties.
Meanwhile, a microcrystalline high-density magnet, produced by an HDDR process, not only has as high a percentage of a hard magnetic phase as a sintered magnet but also has its properties deteriorated to a much lesser degree than a sintered magnet because the magnet of the former type has fine crystal grains with a size of 0.1 μm to 1 μm.
However, even such a microcrystalline high-density magnet obtained by an HDDR process would achieve poor productivity if such a magnet were produced by the manufacturing process in which the HDDR powder is aligned under a magnetic field and then turned into a bulk by a hot compaction process such as hot pressing as disclosed in Patent Documents Nos. 3 to 9. As a result, the manufacturing cost would increase and it would be difficult to mass-produce such magnets at a cost that is low enough to make general-purpose motors.
According to the manufacturing process disclosed in Patent Document No. 10, the size of the main phase is reduced by subjecting the sintered body to the HDDR process. In the HDDR process, however, the volume varies during the HD reaction or the DR reaction. For that reason, when subjected to the HDDR process, the sintered body easily cracks and cannot be produced at a high yield. Also, since a bulk body (sintered body) that has already had its density increased is subjected to the HDDR process, hydrogen, which is an essential element for the HD reaction, will have its diffusion path limited. As a result, the homogeneity of the texture would decrease in the resultant magnet or it would take a lot of time to get the process done. Consequently, the size of the magnet that can be made would be restricted.
According to Patent Document No. 11, by performing a DR process at a temperature of 1,000° C. to 1,150° C., the density of the resultant magnet can be increased without increasing the size of the fine crystal grains and better magnetic properties than a normal R—Fe—B based sintered magnet should be achieved. However, the present inventors discovered and confirmed via experiments that when a sintering process was carried out at 1,000° C. or more in the DR process, it was difficult to increase the density while keeping the crystal grains size so small but abnormal grain growth occurred noticeably. As a result, the magnetic properties eventually deteriorated more than a normal sintered magnet (see Table 2 and Comparative Example 1 to be described later).